Process for the production of coldrolled steel plate having good shape-fixability



Feb. 17 1970 MINEO SHIMIZU ET L 3,4 ,032

PROCESS FOR THE PRODUCTION OF COLD-ROLLED STEEL PLATE HAVING GOOD SHAPE-FIXABILITY Filed Nov. '28, 1966 F I G. 2

FIG.

n m 81' mm f .m mm w mm mp illiiilli illlllilii 600 650 700 750 800 III! Tempemiure range of the present invention w 9 8 7 oz 3 P illlllllllilllll'llll'lll 600 650 700 750 800 z 2 w 2. s

Anneoling iemperurure (C) Coiling iempercnure' of hot-rolled coil (C) I N VE IV 701% M/nea- Shim/'20 M/noru Kawaharada Sh/bafa Masaa/r/ Haruo a "Z @Z United States Patent 3,496,032 PROCESS FOR THE PRODUCTION OF COLD- ROLLED STEEL PLATE HAVING GOOD SHAPE-FIXABILITY Mineo Shimizu, Minoru Kawaharada, Masaaki Shibata, and Haruo Shimada, Kitakyushu, Japan, assignors to Yawata Iron & Steel Co., Ltd., Tokyo, Japan Filed Nov. 28, 1966, Ser. No. 597,255 Claims priority, application Japan, Nov. 30, 1965, 40/73,542 Int. Cl. C21d 7/14, 7/10; C22c 39/50 US. Cl. 14812 2 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a process for producing a cold-rolled steel plate having a good shape-fixability together with a non-aging property, that is, a steel material for providing a press formed article having excellent finished dimensions and accuracy even by press forming at a normal temperature and having no defects, such as, elastic hollow and incomplete rigidity.

It is well known that a steel strip to be press formed is required to have a good workability and causing no rupture, that is, a good press formability is required in such a steel strip. Further, the steel strip is required to have such property that it maintains, after press formed, its desired shape, that is, after removing the punch or die for press forming the steeel strip, the press formed steel strip maintains the same shape as that of the punch or die, or the steel strip is required to have a so-called good shape-fixability. Such a property is of course influenced by the art of conducting press forming, but as to the properties of material, it is considered to be necessary for the good press formability that the plastic strain ratio is large, a work hardening index is large and an elongation is large. Moreover, it is known to be necessary for a good shape-fixability that an yield point is low and a modulus of elasticity is large.

Furthermore, even though a steel plate or press forming has a low yield point directly after the production thereof, if it has a strain aging property the yield point is increased during a time elasping from the skin pass at the production thereof to the press working. Accordingly, in the case of producing a steel plate having a low yield and a good shape-fixability, it must be considered to lower the yield point of the steel plate directly after the production thereof and to afford a non-aging property to the steel plate so as to suppress the increase of the yield point of the steel plate after the production thereof.

By affording a non-aging property to a steel plate, the formation of stretcher strain at press working can be prevented, which is particularly desirable for articles required to have a shape fixability detesting the formation of stretcher strain.

"Processes for producing non-aging steel plates have been known for a long time, that is, there are a process for conducting denitrification and decarburization and a process for stabilizing nitrogen and carbon by adding 3,496,032 Patented Feb. 17, 1970 addition elements. The former, that is, the process for conducting denitrification and decarburization has been employed in an open coil annealing system, but in this process the production cost is high. The latter, that is, the process for adding addition elements has employed for producing a killed steel added with aluminum 'but the blooming yield and the surface conditions of the killed steel are bad as well as the production cost is high. Furthermore, as other example of the latter process has been known a process for producing a non-aging steel plate by adding vanadium. Such a type of steel plate has hitherto aimed at the prevention of the regeneration of yield point elongation by aging to prevent the formation of stretcher strain and the prevention of the reduction of ductility by aging but no consideration has been paid about the value of yield point. If a steel plate is non-aging, the increase of the yield point by strain aging after skin pass rolling does not occur, but as mentioned above, a steel plate having a good shape-fixability cannot be obtained unless the yield point of the steel also in the non-aged state directly after skin pass rolling is made lower than those of general materials.

An object of the present invention is to provide a process for producing a good shape-fixability steel plate having a non-aging property and a yield point reduced immediately after the production thereof improving a conventional steel plate added with vanadium.

Other objects of the present invention will become apparent from the following description referring the accompanying drawings, in which FIG. 1 is graphs showing the relations of the coiling temperature of a hot-rolled coil and the aged amount, the yield point and the grain size and FIG. 2 is graphs showing the relations of the annealing temperature and the aged amount, the yield point and the grain size.

The main composition of the steel used in this invention consists of below 0.12% of C, below 0.6% of Mn, 0.01-0.15 of V, balance being Fe and unavoidable impurities.

The steel having the above composition is formed into the ingot of rimmed steel, semi-killed steel or capped steel (both mechanical and chemical capped steels are called capped steel in this specification) and, is necessary, the molten steel may be subjected to a deoxidation treatment before the addition of vanadium. In the case of conducting deoxidation, the amount of Si or Al used in the deoxidation must be so regulated that killed steel may not be produced, that is, the amount of Si remaining in the steel must be less than 0.08% and the amount of Al must be 0.02% If the amount of Si or Al as the deoxidizing agent remains in the steel in an amount more than the above-defined amount, the mechanical prop erties and cleanness of the steel are deteriorated, and the growth of crystalline grains is obstructed.

Further, if decarburization and deoxidation of the steel are conducted by subjecting it to degassing under a reduced pressure before the addition of vanadium in the ingot making step of molten steel, there is an advantage that the amount of V can be saved.

After the steel having the above composition has been formed into the ingot of rimmed steel, semi-killed steel or capped steel, the ingot is bloomed and then hot-rolled into a plate. The hot-rolling is conducted by conventional conditions but in this invention, the coiling temperature is important, that is, the steel plate must be coiled up within the temperature range of 760-680 C.

The hot-rolled steel plate coiled at the above-mentioned temperature is further cooled, pickled and cold-rolled. The cold-rolled plate is then subjected to recrystallization annealing at the temperature range of 630-730 C. Thus, by

0.12% since if the content of C is above 0.12% the steel is hardened and the yield point thereof is increased, whereby the formability and shape-fixability of the steel are reduced. On the other hand, the lower limit of the content of C is not particularly limited and may be suitably selected depending on the decarburization faculty. :If the content of Mn is above 0.6%, the steel is hardened and the surface contamination becomes remarkable. On the other hand, the lower limit of Mn is not defined, however even if Mn is added in such an amount to be included as one of impurities, it can attain the object of this invention.

V is added for fixing N which is included as an impurity element in the steel, and taking into consideration of the effective yield thereof, the content of V is defined to be 0.01-0.15%. If the content of V is less than 0.01% the effect of V is insufiicient and if it is more than 0.15% the effect corresponding to the added amount of V is not obtained.

About the coiling temperature of a vanadium-added steel plate at hot-rolling, S. Epstein et al. (Journal of Metals; vol. 2, 830 (1950)) teach that the high-coiling temperature of above 1400 F. (760 C.) is better. On the other hand, W. R. D. Jones et al. (Journal of the Iron and Steel Institute; vol. 174, 9 (1953)) teach that if the steel after hot-rolling is not coiled at a low temperature by cooling it with spray water, the surface crystal grains are coarsened to reduce the quality of the steel. However, in such known processes, there is a drawback that the steel has an aging property.

As the results of the broad investigation about the influences of coiling temperature, the inventors have found that in order to obtain both characteristics of low yield point and non aging property, the coiling temperature of 680-760 C., particularly, 700-750 C. is most suitable.

In order to reduce the yield point, it is necessary to coarsen the crystal grain. It is profitable to prepare the crystal grain of a hot-rolled plate coarsened, because, if so, also the crystal grains of the steel plate after coldrolling and annealing can be coarsened. In order to coarsen the crystal grains of the hot-rolled plate, it is desirable that the coiling temperature is higher. However, if the coiling temperature is above 760 C., although the crystal grains are coarsened, cementite is precipitated in the steel in massive state and the influence thereof remains after subsequent cold-rolling and annealing. That is, in such a case, the precipitation of carbon in a solid solution state is prevented at cooling after annealing and hence aging property based on carbon appears. On the other hand, if the coiling temperature is lower than 680 C., the crystal grains do not grow sufliciently in the hot-rolled plate. Further, since sufficient amount of vanadium nitride is not precipitated in the hot-rolled steel plate in such a case, the above-mentioned vanadium nitride must be precipitated in the subsequent annealing step after cold-rolling. However, inthis case, the annealing period of time must be prolonged markedly, which makes the process uneconomical. Furthermore, it has been found that even if the annealing period is prolonged, the crystal grains scarcely grow. After the hot-rolled steel plate has been :ooled, it is pickled, cold rolled and then annealed according to conventional manners. If the steel plate is ansealed at a temperature higher than that conventionally employed in a usual batch type annealing for increasing the grain sizes of the crystals, for example, it is annealed at 750 C., cementite is precipitated in massive state at :he steel plate after cooling is increased and at the same :ime the content of nitrogen in solid solution state is also ncre se A cordingly, in spite ef adding vanadium, thus obtained steel plate shows the same aging property as in usual rimmed steel in such a case. Since such an aging property of the steel plate occurs when the annealing temperature is above 730 C., the annealing temperature is reduced to lower than 730 C. preferably 710 C. On the other hand, even if the steel plate is coiled at a high temperature, the yield point is not reduced if the annealing is conducted at a temperature lower than 630 C. and in this case an extremely long period of time is required for forming stable vanadium nitride, which makes the economical practice of this invention unsuitable. Hence, the lower limit of the annealing temperature is defined to be 630 C. preferably 670 C.

There are no particular limitations in the atmosphere for annealing but in the case of using an oxidizing gas atmosphere, a specific care must be taken for preventing the surface of the steel plate from being oxidized, such as, a suitable coating and the like is necessary.

In addition, if the annealing is conducted at a high temperature side of the above range, the annealing period of time must be short and if it is conducted at the low temperature side, the period must be long, but usually the object can be attained within 16 hours.

On the basis of the recognition that by subjecting the vanadium-added steel to suitable heat treatments 21 low yield point and a non-aging property may be simultaneously achieved, the inventors have discovered that by regulating the coiling temperature of a hot-rolled coil to a temperature range of 680 to 760 C., preferably 700 to 750 C., then conducting a recrystallization softening annealing at a temperature range of 630 to 730 C., preferably 670 to 710 C. after subjecting the said hot-rolled coil to conventional heat treatments covering cooling, pickling and cold-rolling, a steel plate having an excellent shape-fixability can be obtained.

The invention will then be explained referring to the following examples.

EXAMPLE 1 The steel from a convertor having the chemical composition shown in Table l was formed into ingot, bloomed and rolled into a slab, and the slab Was hot-rolled at 900860 C. into a plate having a thickness of 2.5 mm. The hot-rolled steel plate was coiled as hot rolled at various temperatures within the range of 600-820 C., after cold-rolling, annealed for 4 hours at various temperatures within the range of 600-780 C., and then subjected to a skin pass rolling of a reduction rate of 1% into a final steel plate having 0.8 mm. in thickness. About thus obtained steel plates, grain sizes, yield points, and the increased amount of yield point when the plate was artificially aged for 1 hour at C. were measured.

Table 1 Chemical composition, percent:

C 0.07 Si 0.01 Mn 0.35 P 0.013 S 0.015 A1 0.005 V 0.042 N 0.0028

The effect of each coiling temperature after hot-rolling is shown in FIG. 1. In each case, the annealing temperature after cold rolling was 690 C. The effect of each annealing temperature after cold-rolling is shown in FIG. 2. In each case, the hot-roll coiling temperature was 720 C.

As is clearly seen from FIG. 1 and FIG. 2, it was confirmed that by coiling a hot-rolled coil vanadiumadded steel as hot-rolled at a temperature of 680-760 C., preferably 700-750 C. and then annealing it at a temperature of 6307 30 C., preferably 670 710 C.,

after cold-rolling a excellent good shape-fixability steel plate having both characteristics of low yield point and nonaging property can be obtained.

EXAMPLE 2 The steel from a converter having the chemical composition shown in Table 2 was formed into ingot, bloomed and rolled into a slab.

The hot-rolled vanadium-containing slab was coiled as hot-rolled at 720 C. as in Example 1. Thereafter, the coil was cooled, annealed for 4 hours at 700 C. and then subjected to a skin pass rolling of a reduction rate of 1% into a steel plate of 0.8 mm. in thickness.

The mechanical properties of the vanadium-added steel are shown in Table 3 together with those of a usual rimmed steel containing no vanadium (other components are same).

As is clear from the table, the vanadium-added steel produced by the process of this invention is very low in yield point, has non-aging property, and other mechanical properties thereof are good as compared with those of usual rimmed steel.

What is claimed is:

1. A process for the production of a cold-rolled steel plate which comprises the steps of forming an ingot of steel into a slab consisting of below 0.12% by weight of C, below 0.6% of Mn. 0.010-0.15% of V and the balance being Fe and unavoidable impurities; hot-rolling the slab; coiling the hot-rolled plate produced from the hot-rolling step and cooling the plate; pickling the hotrolled plate; cold-rolling the plate; annealing the coldrolled plate; subjecting the annealed plate to a skin pass rolling, wherein the said coiling of the hot rolled plate is conducted at a temperature of 680 C. to 760 C. and the annealing step is conducted at a temperature of 630 C. to 730 C., whereby the finally produced steel plate is characterized by having a good shape-fixability, good nonaging properties, and a low yield point.

' 2. A process according to claim 1 wherein the coiling step is conducted at a temperature between 700 C. and 750 C. and the annealing step is carried out at a temperature between 670 C. and 710 C.

TABLE 3.MECHANICAL PROPERTY Tensile Erichsen Yield point Aging amt. strength Elongation valu (A) (kg .lmmfl) (kg/mm?) (kg/mm?) (percent) (mm.) (mm) V-added steel 14. 9 0. 1 31, 2 48. 3 11. 2 37. 2 Usual rimmed Steel 21. 3 6. 3 33. 1 45. 0 l0. 6 37. 9

*Inereased amount of yield point by an artificial aging of 1 hour at C. (A) Conical eup value.

References Cited UNITED STATES PATENTS 2,878,151 3/1959 Beall et al. 14812 3,239,390 3/1966 Matsukura et a1. 148-12 L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner 

